The present invention relates to methods and apparatus for predicting and gauging the depth of available energy in a battery or electrochemical cell system. More particularly, although not exclusively, the present invention relates to methods and apparatus for measuring battery capacity, charge remaining and reserve time.
It has long been recognised that battery or cell capacity depends on a number of factors. These include the batteries composition, geometry, discharge rate (i.e. load current), age, environmental temperature, end voltage, service history (i.e. characteristics of the batteries last charge), discharge depth and time on float. The available capacity of a battery can be represented as a complex, non-linear function of these parameters. The direct measurement of a number of these parameters to determine battery capacity is either impractical or financially prohibitive. One of the most well known techniques for measuring the capacity of a battery is known as a xe2x80x9cdischarge testxe2x80x9d (IEEE Std 1188-1996; xe2x80x9cIEEE Recommended Practice for Maintenance, Testing, and Replacement of Valve-Regulated Lead-Acid (VRLA) Batteries for Stationary Applicationsxe2x80x9d). This procedure involves the full discharge of a battery into a stable load. A major disadvantage of this approach is that, in the context of the batteries application (for example in a telecommunication system), the system being powered is vulnerable to power outages as the battery must be disconnected from the system during its complete discharge. Further disadvantages include the necessity for bulky external loads, the need for backup power supplies and the labour involved in setting up and supervising the testing procedure.
Other techniques for measuring the battery capacity use methods whereby parameters such as impedance, (Gary J. Markle, xe2x80x9cAC Impedance Testing for Valve Regulated Cell,xe2x80x9d INTELEC 1992, 9-4), conductance, (Michael E. Troy et al, xe2x80x9cMidpoint Conductance Technology Used in Telecommunication Stationary Standby Battery Applications. Part VI. Considerations for Deployment of Midpoint Conductance in Telecommunications Power Applications,xe2x80x9d INTELEC 1997, 29-4), or internal resistance (Isamu Kurisawa and Masashi Iwata, xe2x80x9cInternal Resistance and Deterioration of VRLA Batteryxe2x80x94Analysis of Internal Resistance Obtained by Direct Current Measurement and its Application to VRLA Battery Monitoring Technique,xe2x80x9d INTELEC 1997, 29-3: Glenn Alber and Marco W. Migilaro, xe2x80x9cImpedance Testingxe2x80x94s it a Substitute for Capacity Testing,xe2x80x9d INTELEC 1994, 10-1: Katsuhiko Yamamoto et al, xe2x80x9cDeterioration Estimation Method for 200-Ah Sealed Lead-Acid Batteries,xe2x80x9d NTT Review Vol. 7, No. 4, July 1995) are correlated with a capacity. These latter methods generally employ a composite model based on a number of parameters. This model usually incorporates reference to cell resistance or impedance in determining the battery capacity (Jean Paul Cun et al, xe2x80x9cThe Experience of a UPS Company in Advanced Battery Monitoring,xe2x80x9d INTELEC 1996, 22-5: Petrick K. Ng et al, xe2x80x9cEvaluation of a Reverse Time Prediction Algorithm for Lead Acid Batteryxe2x80x9d, INTELEC 1996, 616-21). These methods have generally been developed for off-line applications and require the use of specialised equipment. Although they have had some success, it is generally considered in the art that these techniques are best suited for identifying gross faults (Michael E. Troy et al, xe2x80x9cMidpoint Conductance Technology Used in Telecommunication Stationary Standby Battery Applications. Part VI. Considerations for Deployment of Midpoint Conductance in Telecommunications Power Applications,xe2x80x9d INTELEC 1997, 29-4: Katsuhiko Yamamoto et al, xe2x80x9cDeterioration Estimation Method for 200-Ah Sealed Lead-Acid Batteries,xe2x80x9d NTT Review VoL. 7, No. 4, July 1995), tracking battery age and making battery life time predictions (Gary J. Markle, xe2x80x9cAC Impedance Testing for Valve Regulated Cell,xe2x80x9d INTELEC 1992, 9-4: Katsuhiko Yamamoto et al, xe2x80x9cDeterioration Estimation Method for 200-Ah Sealed Lead-Acid Batteries,xe2x80x9d NTT Review VoL. 7, No. 4, July 1995). A detailed short-term test of battery capacity measurement is still most effectively produced by the discharge test. Referring again to the latter models discussed above, such models used for on-line capacity measurements are often specific to particular cells and rely on measured parameters (Isamu Kurisawa and Masashi Iwata xe2x80x9cCapacity Estimating Method of Lead-Acid Battery by Short- time Dischargexe2x80x9d, INTELEC 1997, 483-90). Such techniques are therefore susceptible to measurement errors. Further, the number of parameters necessary to classify an entire battery operation can become excessive making such approaches cumbersome and computationally complicated.
Alternative techniques for determining battery capacity have been proposed which are based on either open circuit voltage or charge accumulation (Minoru Kozaki, and Toshihiko Yamazaki, xe2x80x9cRemaining Battery Capacity Meter and Method for Computing Remaining Capacity,xe2x80x9d U.S. Pat. No. 5,691,078, Nov. 25, 1997).
In the context of telecommunications applications, the open circuit method is undesirable. Disconnecting the battery string from the power supply system would leave the telecommunication system vulnerable to switch failure and hence accidental isolation of the string from the system. Further, the charge accumulation approach requires long term monitoring of the battery (or battery string) and is dependent on knowing an accurate initial value of the battery capacity. Any initial error would affect the results of the rest of the monitoring activity. For this reason, this latter approach is considered unreliable.
It is accordingly an object of the present invention to provide a method and apparatus which allows for an accurate measurement of a batteries charge remaining (within the constraints of the application to which the battery is to be put), which avoids or at least ameliorates a number of the above mentioned problems, or at least provides the public with a useful choice.
In one aspect, the present invention provides for a method of testing/characterising one or more cells including the steps of:
obtaining a plurality of data points representing the direct relationship between voltage and charge remaining during an initial discharge of one or more cells; and,
parameterising the data points to obtain a function representing voltage and charge remaining, the function terminating at an end voltage corresponding substantially to a voltage level at which the cell(s) is/are considered to be exhausted, and whereby during a subsequent discharge the function allows the charge remaining to be estimated directly from the cell(s) voltage.
After the parameterisation there may be a further step of calculating the discharge reserve time. Preferably the discharge reserve time is calculated by dividing the charge remaining by either a constant power discharge rate or a constant current discharge rate.
Additionally there may be yet a further step whereby a fully charged cell or cells is subjected to a partial discharge, the charge released during the partial discharge being added to the charge remaining to obtain a measurement of capacity.
Preferably the partial discharge is one that is long enough to avoid the Coup de Fouet region, but is much shorter than a complete discharge of the cell or cells.
The steps can correspond to measuring the cell(s) voltage and current over specific time intervals.
The parameterisation can be effected by means of collecting data points equidistant in the voltage domain, a least squares fit, interpolation and/or extrapolation, or an analytical approach adapted to target the best fit to the data points.
A number of data points for a set level of measurement accuracy can be obtained and parameterised.
Preferably the data points are selected over intervals selected so as to minimise the errors inherent in the parameterisation process.
Preferably any or all of the steps may be repeated due to changes in cell characteristic from ageing, environmental and usage conditions.
Preferably the decision to repeat the steps is determined by comparison of a state of change of the cell(s) derived from a previous test and the actual state of charge of the cell(s).
In a further aspect, the invention provides for a battery charge remaining and capacity measurement and discharge reserve time prediction device including:
a voltage measurement means adapted to measure the voltage of a cell or cells;
a current measurement means adapted to measure the present load on the cell or cells;
a timing means adapted so that a substantially simultaneous measurement of voltage, current and time in respect of the cell or cells can be performed, thereby allowing the collection of a plurality of data points relating to the direct relation between cell voltage and charge remaining; and
a processing means adapted to produce a curve/function directly relating charge remaining to cell voltage during an initial discharge of one or more cells, the curve/function terminating at an end voltage corresponding substantially to a voltage level at which the cell(s) is/are considered to be exhausted, and whereby during a subsequent discharge the curve/function allows the processing means to estimate charge remaining directly from the cell(s) voltage.
The device can include a microprocessor adapted to manipulate the voltage, time and current to provide data points representing the voltage as a function of charge remaining wherein the charge remaining is expressed in amp/hours.
The device can be adapted to calculate the discharge rate of a cell or cells, the device using the charge remaining and a discharge rate to determine the discharge reserve time. Preferably the discharge rate is calculated for either constant power or constant current discharge, and the discharge reserve time is expressed in hours and fractions of an hour.
The device can include a discharge means, the discharge means being adapted to at least partially discharge a cell or cells and measure the charge released from said cell or cells during the discharge, the cell or cells capacity being derived from the charge released during the discharge and the charge remaining.
The device may incorporate an output means adapted to graphically, numerically or otherwise indicate, in real time, the charge remaining, capacity measurement and/or discharge reserve time of the cell or cells being measured.
Preferably the device includes a means adapted to, at the initiation of a user, measure the data points and effect a parameterisation automatically.
The device can be further adapted to incorporated means for sensing variations in the environmental conditions in which the cell or cells are used and be further adapted so that in response to predetermined criteria, the device remeasures the data points and establishes an updated parameterisation.
Preferably the device may output the charge remaining, capacity measurement and/or discharge reserve time of the cell or cells constantly.
Alternatively the charge remaining, capacity measurement and/or discharge reserve time may be output in response to a user activation or request.